This invention relates to acrylate-based adhesives and in particular plasticized acrylate-based pressure sensitive adhesives.
Pressure-sensitive adhesives are typically coated from solvent or water, extruded as hot-melts, or they are bulk polymerized directly on a release liner or a backing. As more environmentally friendly coating techniques are sought, solvent-and water-free processes have become increasingly desirable.
Today, hot-melt coating is used for pressure-sensitive adhesives based on styrene/rubber based block-copolymers, natural rubbers and acrylate polymers. Block copolymers and natural rubbers can be handled as dry polymer beads or crumbs, which are compounded with tackifiers and oils in the extruder to make the pressure-sensitive adhesives.
Typically, acrylic polymers used for pressure-sensitive adhesives are inherently tacky, which can make the polymer handling and feeding to the equipment challenging. Today, acrylic pressure-sensitive adhesives are delivered as remeltable polymers in drums or pails, extrudable adhesives packaged in siliconized boxes or small pillows and pouches of sticky polymer enclosed in a non-tacky polymer shell. All of these formats require special packaging equipment at the adhesive manufacturing site and special equipment for handling/feeding the polymer at the coating site.
Most of the commercial block copolymer and natural rubber pressure sensitive adhesives use combinations of the elastomer and additives, such as tackifiers and plasticizing agents (or oils). Alternatively, acrylic pressure-sensitive adhesives are often free of any of these additives. Proper selection of the (meth)acrylate and copolymerizable comonomers allows the manufacture of acrylic pressure-sensitive adhesive polymers that are inherently tacky. The absence of additives and the saturated backbone of the acrylate polymer are very common reasons for the selection of acrylic pressure-sensitive adhesives for outdoor applications or when durable bonds need to be made.
However, it is not always possible to achieve the desired pressure-sensitive adhesive properties using acrylic polymers without the use of additives. For example, in order to enhance adhesion against low surface energy substrates, acrylic pressure-sensitive adhesives can be formulated with tackifiers. Since acrylic pressure sensitive adhesives have glass transition temperatures (Tg) that are typically below 0xc2x0 C., tackifiers are commonly used to decrease the shear storage modulus of the acrylic polymer to improve its surface wetting characteristics. The tackifiers typically have softening points that are well above room temperature, so the Tg of the compounded adhesive will also increase over the untackified acrylate polymer. This effect typically limits the useful amount of tackifiers in a tackified acrylate pressure sensitive adhesive formulation to about 60 pph or less relative to the elastomer.
Tackifiers are often colored and oxidatively unstable, thus sacrificing the unique clarity and stability for which acrylics are known. In addition, it has been found that most commercial tackifiers that are compatible with the polar acrylate matrix will also interfere with bulk polymerization, resulting in much lower molecular weights for the resultant polymer.
Plasticizing agents are less commonly used in acrylate pressure-sensitive adhesives, as plasticizing agents typically decrease the Tg of the compounded adhesive, resulting in a loss of peel strength. Actually, plasticizing agent migration into acrylic pressure sensitive adhesives is one of the well-known failure mechanisms in applications against plasticized polyvinyl chloride.
Briefly, in one aspect of the present invention essentially non-tacky, polymers are plasticized into pressure-sensitive adhesives. Plasticized polymer based pressure-sensitive adhesives of the present invention comprises:
a. about 100 parts by weight of a base copolymer having a Tg greater than about 0xc2x0 C., wherein the base copolymer is formed from and comprises:
(1) about 50 to 70% by weight of a high Tg comonomer component, wherein the homopolymer formed from the high Tg comonomer component has a Tg of at least about 20xc2x0 C.;
(2) optionally, up to about 20% by weight based on the total weight of the base copolymer of an acidic comonomer; and
(3) about 30 to 50% by weight of one or more low Tg (meth)acrylate comonomer, wherein the Tg of the homopolymer of the low Tg comonomer is less than about 20xc2x0 C., and
b. about 1 to about 100 parts based on the base copolymer of a non-reactive, non-volatile, non-acrylic-based plasticizing agent.
The plasticized pressure-sensitive adhesives of the present invention have several general advantages over conventional acrylate pressure-sensitive adhesive compositions. Since the base copolymer exhibits little or no tack prior to its combination with the plasticizing agent, new means for handling, processing and delivering these compositions can be practiced. The base copolymer can be transported and processed similarly to other elastomeric materials used in pressure-sensitive adhesive compositions, without the special packaging and processing generally required with traditional acrylate pressure-sensitive adhesives. When the base copolymer is in particulate form, such as dried particles prepared using suspension, emulsion or dispersion polymerization methods or cryo-grinding, powder handling and coating techniques can be employed and exploited to create new pressure-sensitive-adhesive materials and new product applications. Furthermore, the pressure-sensitive properties of the composition can be activated when desired by the formulator or when required by the application.
Conventional understanding of the effect of the addition of plasticizing agents to pressure-sensitive adhesives is that adhesive performance degrades following plasticization. However, by combining the plasticizing agent with a high Tg, high shear storage modulus base copolymer, stable and useful pressure-sensitive adhesive materials can readily be formulated.
In addition, it has been found that some specific classes of plasticizing agent show very limited interference with radiation crosslinking and polymerization processes. Since plasticizing agents are typically free of unsaturation, they are as oxidatively stable as the base copolymer. This inertness toward radiation opens some novel opportunities for cured adhesives where the plasticizing agents can be used as an alternative for tackifiers. Tackifiers are known to interfere with UV processing. Plasticizing agents are also generally significantly lower in cost than tackifiers and can be selected to be compatible and non-volatile with respect to the base copolymer.
In addition, with regard to tackifying agents, it has been found that some tackifiers can cause allergenic response in some medical adhesive applications. Use of plasticizing agents along with a base copolymer to form plasticized pressure sensitive adhesives according to the present invention can offer alternative routes towards modification of base copolymer properties to yield non-allergenic compositions for skin and other medical adhesive applications.
Advantageously, the adhesives of the present invention exhibit fluid-like characteristics despite generally high molecular weights of the base copolymer. Fluidity of the plasticized pressure-sensitive adhesive compositions occurs at lower temperatures as compared to the unplasticized base copolymer. Thus, the adhesives of the present invention provide a distinct advantage for applications using heat sensitive substrates because coatable viscosities can be obtained at lower melt temperatures. Furthermore, because the adhesives of the present invention have a low melt viscosity, they tend to retain less orientation and relax more readily during extrusion. Lower melt viscosities also permit faster coating speeds, an important consideration in the production and commercialization of articles coated with the plasticized pressure-sensitive adhesive composition.
The addition of plasticizing agents to the pressure-sensitive adhesives of the present invention can also provide other processing advantages. In waterborne coatings, the plasticizing agent may enhance film formation of the plasticized pressure-sensitive adhesive by promoting coalescence of the drying emulsion. If solventless polymerization methods are desired, the plasticizing agent can also serve as a non-volatile xe2x80x9csolventxe2x80x9d for immiscible or incompatible mixtures of high Tg comonomers, low Tg (meth)acrylate comonomers and optional acidic comonomers. Such incompatible comonomer mixtures typically require a volatile reaction medium, such as an organic solvent or water, to promote effective copolymerization. Unlike these volatile reaction media, the plasticizing agent does not have to be removed to make a pressure sensitive adhesive.
In another aspect of the present invention, a method of coating a plasticized polymer based pressure sensitive adhesive comprises the steps of:
a. admixing about 100 parts by weight of a base copolymer having a Tg greater than about 0xc2x0 C. and about 1 to about 100 parts of a non-reactive, non-volatile, non-acrylic-based plasticizing agent, wherein the base copolymer is formed from:
(1) about 50 to 70% by weight of a high Tg comonomer component, wherein the homopolymer formed from the high Tg comonomer component has a Tg of at least about 20xc2x0 C.;
(2) optionally, up to about 20% by weight based on the total weight of the base copolymer of an acidic comonomer; and
(3) about 30 to 50% by weight of one or more low Tg (meth)acrylate comonomer, wherein the Tg of the homopolymer of the low Tg comonomer is less than about 20xc2x0 C., and
b. coating the admixture prepared according to part (a) onto a substrate from a solvent or aqueous solution.
This method of coating can be accomplished using various conventional coating techniques, such as knife and blade coating, as well as die and curtain coating.
In an alternative coating method, a plasticized polymer based pressure sensitive adhesive is hot-melt coated comprising the steps of:
a. admixing about 100 parts by weight of a base copolymer having a Tg greater than about 0xc2x0 C. and about 1 to about 100 parts of a non-reactive, non-volatile, non-acrylic-based plasticizing agent at a temperature sufficient to melt and thoroughly mix the base copolymer and plasticizing agent , wherein the base copolymer is formed from:
(1) about 50 to 70% by weight of a high Tg comonomer component, wherein the homopolymer formed from the high Tg comonomer component has a Tg of at least about 20xc2x0 C.;
(2) optionally, up to about 20% by weight based on the total weight of the base copolymer of an acidic comonomer; and
(3) about 30 to 50% by weight of one or more low Tg (meth)acrylate comonomer, wherein the Tg of the homopolymer of the low Tg comonomer is less than about 20xc2x0 C., and
b. hot melt coating the admixture prepared according to part (a) onto a substrate.
In another aspect of the present invention, the plasticized polymer based pressure sensitive adhesive using radiation polymerization methods comprising the steps of:
a. preparing a prepolymeric syrup comprising:
(1) about 50 to 70% by weight of a high Tg comonomer component, wherein the homopolymer formed from the high Tg comonomer component has a Tg of at least about 20xc2x0 C.;
(2) optionally, up to about 20% by weight based on the total weight of the base copolymer of an acidic comonomer; and
(3) about 30 to 50% by weight of one or more low Tg (meth)acrylate comonomer, wherein the Tg of the homopolymer of the low Tg comonomer is less than about 20xc2x0 C., and
b. combining the prepolymeric syrup with about 1 to 100 parts based on 100 parts of the sum of components a(1) to a(3) of a non-reactive, non-volatile, non-acrylic-based plasticizing agent to form a polymerizable mixture; and
c. exposing the polymerizable mixture to radiation sufficient to polymerize the polymerizable mixture and to form the plasticized polymer based pressure sensitive adhesive.
Alternatively, the radiation polymerized plasticized polymer based pressure sensitive adhesive can be prepared by combining the non-reactive, non-volatile, non-acrylic-based plasticizing agent with the comonomers to form a prepolymeric syrup, then exposing the prepolymeric syrup to radiation.
Due to the distinct advantage of using powdered components to prepare a pressure sensitive adhesive, a method of coating a plasticized polymer based pressure sensitive adhesive comprising the steps of:
a. admixing about 100 parts by weight of a powdered base copolymer having a Tg greater than about 0xc2x0 C. and about 1 to about 100 parts of a solid, powdered, non-reactive, non-acrylic-based plasticizing agent, wherein the base copolymer is formed from:
(1) about 50 to 70% by weight of a high Tg comonomer component, wherein the homopolymer formed from the high Tg comonomer component has a Tg of at least about 20xc2x0 C.;
(2) optionally, up to about 20% by weight based on the total weight of the base copolymer,of an acidic comonomer; and
(3) about 30 to 50% by weight of one or more low Tg (meth)acrylate comonomer, wherein the Tg of the homopolymer of the low Tg comonomer is less than about 20xc2x0 C., and
b. powder coating the admixture prepared according to part (a) onto a substrate, and
c. heating all or a portion of the coated admixture to a temperature sufficiently high enough to melt the solid, powdered, non-reactive plasticizing agent.
In yet another aspect, a method of coating a plasticized polymer based pressure sensitive adhesive comprises the steps of:
a. powder coating a layer of a powdered base copolymer having a Tg greater than about 0xc2x0 C. onto a substrate, wherein the base copolymer is formed from:
(1) about 50 to 70% by weight of a high Tg comonomer component, wherein the homopolymer formed from the high Tg comonomer component has a Tg of at least about 20xc2x0 C.;
(2) optionally, up to about 20% by weight based on the total weight of the base copolymer of an acidic comonomer; and
(3) about 30 to 50% by weight of one or more low Tg (meth)acrylate comonomer, wherein the Tg of the homopolymer of the low Tg comonomer is less than about 20xc2x0 C., and
b. coating a liquid, non-reactive, non-acrylic-based plasticizing agent over all or a portion of the powdered base copolymer layer such that about 100 parts of the coated base copolymer are plasticized with about 1 to 100 parts of the non-reactive, non-acrylic-based plasticizing agent.
In a related aspect of the present invention, a method of coating a plasticized polymer based pressure sensitive adhesive comprises the steps of:
a. coating a layer of a base copolymer having a Tg greater than about 0xc2x0 C. onto a substrate, wherein the base copolymer is formed from:
(1) about 50 to 70% by weight of a high Tg comonomer component, wherein the homopolymer formed from the high Tg comonomer component has a Tg of at least about 20xc2x0 C.;
(2) optionally, up to about 20% by weight based on the total weight of the base copolymer of an acidic comonomer; and
(3) about 30 to 50% by weight of one or more low Tg (meth)acrylate comonomer, wherein the Tg of the homopolymer of the low Tg comonomer is less than about 20xc2x0 C., and
b. coating a liquid or solid, non-reactive, non-acrylic-based plasticizing agent over all or a portion of the powdered base copolymer layer such that about 100 parts of the coated base copolymer are plasticized with about 1 to 100 parts of the non-reactive, non-acrylic-based plasticizing agent.
As used herein:
xe2x80x9cpressure-sensitive adhesivexe2x80x9d or xe2x80x9cPSAxe2x80x9d refers to a viscoelastic material that possesses the following properties: (1) aggressive and permanent tack, (2) adherence with no more than finger pressure, (3) sufficient ability to hold onto an substrate, and (4) sufficient cohesive strength to be removed cleanly from the substrate.
xe2x80x9ccompatiblexe2x80x9d refers to plasticizing agents that:
1) exhibit no gross phase separation from the base copolymer when blended in the prescribed amounts
2) once mixed with the base copolymer, do not significantly phase separate from the base copolymer upon aging; and
3) function as a rheological modification agent for the base copolymer, such that the plasticized composition exhibits pressure-sensitive properties as defined above.
xe2x80x9cnon-reactivexe2x80x9d refers to plasticizing agents that do not contain free radically reactive ethylenically unsaturated groups that could co-react with the comonomers of the base copolymer or functionalities that significantly inhibit the polymerization of these monomers;
xe2x80x9cnon-volatilexe2x80x9d refers to plasticizing agents that, when compounded with the base copolymer of this invention, generate less than 3% VOC (volatile organic content). The VOC content can be determined analogously to ASTM D 5403-93 by exposing the coated composition to 100+xe2x88x925xc2x0 C. in a forced draft oven for 1 hour. If less than 3% plasticizing agent is lost from the plasticized pressure-sensitive adhesive composition, then the plasticizing agent is considered xe2x80x9cnon-volatilexe2x80x9d.
xe2x80x9csolventlessxe2x80x9d refers to adhesive polymerizable mixtures that are essentially 100% solid systems (i.e. essentially free of V.O.C.). Usually, such polymerizable mixtures have no more than about 5% organic solvents or water, more typically no more than about 3% organic solvents or water. Most typically, such polymerizable mixtures are free of organic solvents and water.